Preheat oven to 375 degrees.
Combine the peach chunks, blackberries, and ⅓ cup granulated sugar in the skillet. I used a 10-inch cast iron skillet. You don't have to mix this too much
In another, smaller bowl, combine the flour, ⅔ cup (minus 2 tablespoons), baking powder, and salt. Mix well.
Pour the buttermilk and vanilla over top. Mix until it is mostly smooth.
Pour the mixture over the peaches and blackberries in the cast iron skillet.
Sprinkle the reserved sugar over the cobbler and place in the oven.
Bake until the cobbler is golden brown and the peaches tender. Mine took about 30 minutes. Plan on 25-35 minutes.
You can serve this hot, that is delicious. Or you can allow it to cool slightly before serving. As a note, this is one of my favorite breakfasts!
This is really good served on its own, with vanilla ice cream, or even with whipped cream.
It will hold well for up to 5 days in the fridge.